Low-alloy steel sheets, particularly for vehicle body construction are not corrosion resistant after they have been produced using suitable forming steps, either by means of hot rolling or cold rolling. This means that even after a relatively short period of time, moisture in the air causes oxidation to appear on the surface.
It is known to protect steel sheets from corrosion by means of appropriate corrosion protection coatings. According to DIN 50900, Part 1, corrosion is the reaction of a metallic material with its environment, producing a measurable change in the material, and can impair the function of a metallic part or an entire system. In order to avoid corrosion damage, steel is usually protected so that it resists corrosion-inducing influences for the required length of service life. Corrosion damage prevention can be achieved by influencing the properties of the reaction partners and/or by changing the reaction conditions, by separating a metallic material from the corrosive medium through the application of protective coatings, and by means of electrochemical measures.
According to DIN 50902, a corrosion protection coating is a coating produced on a metal or in the region close to the surface of a metal and is comprised of one or more layers. Multilayer coatings are also referred to as corrosion protection systems.
Possible corrosion protection coatings include, for example, organic coatings, inorganic coatings, and metallic coatings. The reason for using metallic corrosion protection coatings is to lend the steel surface the properties of the coating material for the longest possible period of time. The selection of an effective metallic corrosion protection correspondingly requires knowledge of the corrosion-inducing chemical relationships in the system comprised of the steel, coating metal, and aggressive medium.
The coating metal can be electrochemically more noble or less noble than steel. In the first case, the respective coating metal protects the steel only by forming protective coatings. This is referred to as a so-called barrier protection. As soon as the surface of the coating metal develops pores or is damaged, a “local element” forms in the presence of moisture in which the base partner, i.e. the metal to be protected, is attacked. The more noble coating metals include tin, nickel, and copper.
On the one hand, base metals provide protective covering layers; on the other hand, since they are no more noble than steel, they are also attacked when there are breaches in their coating. If such a coating becomes damaged, then the steel is not attacked as a result, but the formation of local elements begins to corrode the base covering metal. This is referred to as a so-called galvanic or cathodic corrosion protection. The base metals include zinc, for example.
Metallic protective layers are applied by means of a variety of methods. Depending on the metal and the method, the bond with the steel surface is chemical, physical, or mechanical and runs the gamut from alloy formation and diffusion to adhesion and simple mechanical bracing.
The metallic coatings should have technological and mechanical properties similar to those of steel and should also behave similarly to steel in reaction to mechanical stresses or plastic deformations. The coatings should also not be damaged by forming and should also not be negatively affected by forming procedures.
When applying hot dipped coatings, the metal to be protected is dipped into liquid molten metal. The hot dipping produces corresponding alloy layers at the phase boundary between the steel and the coating metal. An example of this is hot-dip galvanizing.
In continuous hot-dip galvanizing, the steel band is conveyed through a zinc bath at a bath temperature of approx. 450° C. The coating thickness—typically 6-20 μm—is adjusted by means of slot nozzles (using air or nitrogen as the stripping medium) that strip off the excess zinc scooped up by the band. Hot-dip galvanized items have a high degree of corrosion resistance and good suitability for welding and forming; they are chiefly used in the construction, automotive, and household appliance industries.
It is also known to produce a coating from a zinc-iron alloy. To accomplish this, these items, after the hot-dip galvanizing, undergo a diffusion annealing at temperatures above the melting point of zinc, usually between 480° C. and 550° C. This causes the zinc-iron alloy layers to grow and the overlying zinc layer to shrink. This method is referred to as “galvannealing”. The zinc-iron alloy thus generated likewise has a high resistance to corrosion, and a good suitability for welding and forming; its chief uses are in the automotive and household appliance industries. Hot dipping can also be used to produce other coatings made of aluminum, aluminum-silicon, zinc-aluminum, and aluminum-zinc-silicon.
It is also known to produce electrolytically deposited metal coatings, which means that metallic coatings comprised of electrolytes are deposited in an electrolytic fashion, i.e. with current passing through.
Electrolytic coating can also be used for metals that cannot be applied using the hot dipping method. Electrolytic coatings usually have layer thicknesses of between 2.5 and 10 μm and are generally thinner than hot-dipped coatings. Some metals such as zinc also permit the production of thick-layered coatings using the electrolytic coating method. Electrolytically galvanized sheets are primarily used in the automotive industry; because of their high surface quality, these sheets are chiefly used to construct the outer body. They have a good forming capacity, are suitable for welding, store well, and have matte surfaces to which paint adheres well.
Particularly in the automotive field, there is a constant push toward ever lighter raw vehicle bodies. On the one hand, this is because lighter vehicles consume less fuel; on the other hand, raw vehicle bodies need to be lighter in order to offset the weight of the ever more numerous auxiliary functions and auxiliary units with which modem vehicles are being equipped.
At the same time, however, safety requirements for motor vehicles are becoming more and more stringent; the vehicle body must assure the safety of the passengers in the vehicle and protect them in the event of an accident. It has therefore become necessary to provide a higher level of accident safety with lighter vehicle body weights. This can only be achieved by using materials with an increased strength, particularly in the region of the passenger compartment.
In order to achieve the required levels of strength, it is necessary to use steel types with improved mechanical properties or to treat the steel types used in order to provide them with the necessary mechanical properties.
In order to produce steel sheets with an increased strength, it is known to form steel parts and simultaneously harden them in a single step. This method is also referred to as “press hardening”. In this process, a steel sheet is heated to a temperature above the austenitization temperature, usually above 900° C., and then formed in a cold die. The die forms the hot steel sheet, which, due to its contact with the surfaces of the cold die, cools very rapidly so that the known hardening effects occur in the steel. It is also known to first form the steel sheet and then cool and harden the formed sheet steel part in a calibration press. By contrast with the first method, this has the advantage that the sheet is formed in the cold state, which makes it possible to achieve more complex shapes. In both methods, however, the heating causes scaling to occur on the surface of the sheet, so that after the forming and hardening, the surface of the sheet must be cleaned, for example by means of sandblasting. Then, the sheet is cut to size and if need be, the necessary holes are punched into it. In this case, it is disadvantageous that the sheets have a very high degree of hardness at the time they are mechanically machined, thus making the machining process expensive, in particular incurring a large amount of tool wear.
The object of U.S. Pat. No. 6,564,604 B2 is to produce steel sheets that then undergo a heat treatment and to create a method for manufacturing parts by hardening these coated steel sheets. In spite of the temperature increase, this approach is intended to assure that the steel sheet is not decarburized and the surface of the steel sheet does not oxidize before, during, or after the hot pressing or heat treatment. To this end, an alloyed, intermetallic mixture is applied to the surface before or after the punching, which should provide protection from corrosion and decarburizing and can also provide a lubricating function. In one embodiment form, the above-mentioned patent proposes using a conventional zinc layer that is clearly applied electrolytically; the intent is for this zinc layer, along with the steel substrate, to transform into a homogeneous Zn—Fe alloy in a subsequent austenitization of the sheet substrate. This homogeneous layer structure is verified by means of microscopic images. This coating should have a mechanical resistance that protects it from melting, thus contradicting earlier assumptions. In practice, however, such a property is not apparent. In addition, the use of zinc or zinc alloys should offer a cathodic protection to the edges if cuts are present. In this embodiment form, however, contrary to the contentions in the above-mentioned patent, a coating of this kind disadvantageously provides hardly any cathodic corrosion protection at the edges and in the region of the sheet metal surface and provides only poor corrosion protection in the event that the coating is damaged.
In the second example in U.S. Pat. No. 6,564,604 B2, a coating is disclosed, which is composed of 50% to 55% aluminum and 45% to 50% zinc, possibly with small quantities of silicon. A coating of this kind is not novel in and of itself and is known by the brand name Galvalume®. According to the above-mentioned patent, the coating metals zinc and aluminum should combine with iron to form a homogeneous zinc-aluminum-iron alloy coating. The disadvantage of this coating is that it no longer achieves a sufficient cathodic corrosion protection; but when it is used in the press hardening process, the predominantly barrier-type protection that it provides is also insufficient due to inevitable surface damage in some regions. In summary, the method described in the above patent is unable to solve the problem that in general, zinc-based cathodic corrosion coatings are not suitable for protecting steel sheets, which, after being coated, are to be subjected to a heat treatment and possibly an additional shaping or forming step.
EP 1 013 785 A1 has disclosed a method for producing a sheet metal part in which the surface of the sheet is to be provided with an aluminum coating or an aluminum alloy coating. A sheet provided with coatings of this kind should be subjected to a press hardening process; possible coating alloys disclosed include an alloy containing 9-10% silicon, 2-3.5% iron, and residual aluminum with impurities, and a second alloy with 2-4% iron and the residual aluminum with impurities. Coatings of this kind are intrinsically known and correspond to the coating of a hot-dip aluminized sheet steel. A coating of this kind has the disadvantage that it only achieves a so-called barrier protection. The moment a barrier protection coating of this kind is damaged or when fractures occur in the Fe—Al coating, the base material, in this case the steel, is attacked and corrodes. No cathodic protection is provided.
It is also disadvantageous that when the steel sheet is heated to the austenitization temperature and undergoes the subsequent press hardening step, even a hot-dip aluminized coating is subjected to such chemical and mechanical stress that the finished part does not have a sufficient corrosion protection coating. This substantiates the view that such a hot-dip aluminized coating is not sufficiently suitable for the press hardening of complex geometries, i.e. for the heating of a steel sheet to a temperature greater than the austenitization temperature.
DE 102 46 614 A1 has disclosed a method for producing a coated structural part for the automotive industry. This method is intended to eliminate the disadvantages of the above-mentioned European patent application 1 013 785 A1. In particular, the contention therein is that by using the dipping method according to European patent application 1 013 785 A, an intermetallic phase would already have been produced during the coating of the steel and that this alloy layer between the steel and the actual coating would be hard and brittle and would fracture during cold forming. As a result, microfractures would occur to such an extent that the coating itself would come loose from the base material and consequently lose its ability to protect. According to DE 102 46 614 A1, therefore, a coating comprised of metal or a metal alloy is applied by means of at least one galvanic coating method in an organic, non-aqueous solution; according to the above-mentioned patent application, aluminum or an aluminum alloy is a particularly well-suited and therefore preferable coating material. Alternatively, zinc or zinc alloys would also be suitable. A sheet coated in this way can then undergo a cold preforming followed by a hot final forming. But this method has the disadvantage that an aluminum coating, even when it has been electrolytically applied, offers no further corrosion protection once the surface of the finished part is damaged since the protective barrier has been breached. An electrolytically deposited zinc coating has the disadvantage that when heated for the hot forming, most of the zinc oxidizes and is no longer available for a cathodic protection. The zinc vaporizes in the protective gas atmosphere.
An object of the present invention is to create a method for producing a part made of hardened steel sheet with an improved cathodic corrosion protection.
A further object of the present invention is to create a cathodic corrosion protection for steel sheets that undergo a forming and hardening.